U.S. Pat. No. 4,648,498 describes a servo control device for modifying the state of the torque coupling device and being carried by a fixed support, the control device being of the kind comprising: an electric motor; a driving member, actuated by the electric motor and being rotatable, but secured against translational movement, with respect to the fixed support; and a driven member, driven by the driving member and being fixed against rotation but capable of translation movement with respect to the fixed support.
The servo device is lodged inside a grooved cartridge which constitutes a tube guide and which is fixed with respect to a radial plate or wheel which is carried on the clutch casing, with the cartridge carrying the electric motor centrally on it. The driving member comprises a lead screw carried by the cartridge, while the driven member comprises a non-rotatable nut which engages with the lead screw.
Such an arrangement, whereby the driven member and driving member are coaxial with each other, is not able to be employed universally, since it cannot be used when the servo device has to be intersected by a shaft. The same is true in the arrangement disclosed in U.S. Pat. No. 4,852,419, in which the driving member and the driven member are not coaxial, which limits the force that can be transmitted.
In order to overcome this disadvantage, it is possible to adopt a hydraulic servo system, as described in French published Patent Application FR-2 165 647A and the corresponding British published Patent Application No. GB 1 379 416A. Such an arrangement is however somewhat costly because of the sealing requirements, and it also calls for the use of expensive electrically operated valves.